An integrated circuit (IC) die (also called chip) is classically put into a protective package acting as a mechanical interface between said integrated circuit die and a printed circuit board (PCB). A very large number of different types of package exist, which are basically separated into two main techniques.
In the traditional wire-bond packaging technique, illustrated in FIGS. 1 and 2, an integrated circuit die 10 is placed into a protective package 12 and bond pads 9 of said integrated circuit die 10 are connected to pins 11 of said protective package 12 via tiny bond wires 13. These pins 11 are in turn connected to tracks 14 of a printed circuit board 15.
In the more recent flip-chip technique, illustrated in FIG. 3, the integrated circuit die 10 is directly connected to the printed circuit board 15 via solder balls 16 (or solder bumps). In this technique, solder balls 16 are placed onto solder pads 17 of the integrated circuit die 10. Then, the integrated circuit die 10 is flipped and placed onto the printed circuit board 15, the solder balls establishing electrical connections between the solder pads 17 and the tracks 14. As can be seen in FIG. 4, such an integrated circuit die 10 comprises an extra metal layer, called redistribution layer (RDL). The redistribution layer comprises redistribution wires 18 connecting the solder pads 17 to the input/output bond pads 14 of the integrated circuit die 10. As it is classically practiced, two bond pads 20, 21 are dedicated to the digital ground (VSS) and the analog ground (AVSS). Inconveniently, the digital ground is particularly noisy because of the high frequency content of digital signals.
Thus, in a mixed digital/analog radiofrequency (RF) integrated circuit packaged with the flip-chip technique, in order to keep the noise from transferring from the noisy digital ground to the analog ground while minimizing the number of solder balls used, a same solder pad 19 is connected to both the VSS bond pad 20 and the AVSS bond pad 21 via two RDL wires 22,23. When a solder ball 30 (cf. FIG.5) is placed onto the solder pad 19 and grounded via a printed circuit board track 24, the AVSS bond pad 21 is isolated from the VSS bond pad 20 at high (radio) frequencies, for instance at 2.4 GHz. It is to be noted that the isolation does not work at low frequencies. This is because each wire 22, 23 has an inductance in the order or a few nano-Henry, which is an important value at Giga-Hertz but would be equivalent to a short at low frequencies.
FIG. 5 shows an example of such a case. As it can be seen, the integrated circuit die 10 comprises a bond pad 25 constituting an input for a signal received by an antenna 26. The antenna 26 is electrically connected to a track 27 of the printed circuit board 15, and said track 27 is electrically connected to the antenna input 25 via a solder ball 28. A low-noise amplifier (LNA) 29 is included on the integrated circuit die 10, so as to amplify the antenna signal. More specifically, a positive terminal of the LNA 29 is connected to the antenna bond pad 25, and a negative terminal of the LNA 29 is connected to the AVSS bond pad 21. Thanks to the isolation of the AVSS bond pad 21 from the VSS bond pad 20 (chip-grounded) via the solder ball 30, only the antenna signal is amplified by the LNA 29, not the noise from the digital ground.
To serve a multitude of applications, integrated circuits may be designed for both flip-chip and wire-bond packaging techniques. However, using the integrated circuit die of FIG.4 with a wire-bond package may be problematic. For instance, FIG.6 shows the integrated circuit die 10 of FIG.5 packaged with a wire-bond technique. In this case, noise isolation is performed via two bond-wires 31, 32 that connect the AVSS bond pad 21 and the VSS bond pad 20 to the track 27 (grounded) of the printed circuit board 15. It is inconvenient that this isolation is degraded by the RDL wires 22, 23 that connect the VSS bond pad 20 and the AVSS bond pad 21, since noise may be transmitted from the VSS bond pad 20 to the AVSS bond pad 21 through these RDL wires 22, 23.